1) Field of the Invention
The present invention relates to a semiconductor laser device, a semiconductor laser module, and an optical fiber amplifier that do not confine an incident light from outside and having a different wavelength from a wavelength of a laser beam emitted.
2) Description of the Related Art
Conventionally, information is transmitted through the Internet or the like, by optical fiber communications. In the optical fiber communications, information is transmitted by transmitting optical signals through optical fibers. Various designs have been proposed to enable long distance transmission of optical signals, but it is currently impossible to reduce losses in light intensity accompanying the long distance transmission to zero, and hence, optical signals attenuate at a certain rate. Therefore, an optical fiber amplifier for amplifying the attenuated optical signals becomes necessary.
FIG. 12A is a diagram of the structure of the optical fiber amplifier of a backward pumping type, of the optical fiber amplifiers according to the related art. An optical signal emitted from a signal light source 101 is transmitted though the optical fiber, and enters into an amplifying optical fiber 104. Pump light emitted from a pump light source 102, on the other hand, is transmitted though a pump light-transmitting optical fiber 105, passes through a coupler 103, and enters into the amplifying optical fiber 104. In an optical fiber amplifier using erbium doped fiber amplifier (EDFA), erbium ions are added to the amplifying optical fiber 104, and since the pump light enters into the amplifying optical fiber 104, the erbium ions are pumped to a high energy state.
When a signal light enters into the amplifying optical fiber 104 in the pumped state, light having the same wavelength and the same phase as those of the signal light is stimulated and emitted. Therefore, the intensity of the signal light is amplified as compared with that of the signal light before entering into the amplifying optical fiber 104, and the signal light is transmitted through the optical fiber as an amplified signal light.
The conventional optical fiber amplifier, however, has a problem in that the signal light includes certain noises due to the presence of the pump light source 102. The coupler 103 optically couples the optical fiber for transmitting the signal light and the pump light-transmitting optical fiber 105. As shown in FIG. 12B, therefore, not only the pump light is transmitted to the optical fiber, but also a part of the signal light indicated by an arrow with a dotted line in FIG. 12B is separated by the coupler 103, transmitted through the pump light-transmitting optical fiber 105, to enter into the pump light source 102.
On the other hand, the semiconductor laser device forming the pump light source 102 includes a resonator structure for effecting laser oscillation. Specifically, in the semiconductor laser device forming the pump light source 102, a high reflection coating is provided on a facet opposite to a laser beam emitting facet. Therefore, a part of the signal light separated by the coupler 103 and entering into the semiconductor laser device is reflected by the high reflection coating and emitted from the pump light source, is transmitted through the pump light-transmitting optical fiber 105, and is coupled again with the signal light amplified by the coupler 103. There is a path difference between the signal light amplified by the amplifying optical fiber 104 and the part of the signal light entering into the pump light source, by the pump light-transmitting optical fiber 105 and the length of the resonator of the pump light source 102. Therefore, the signal light output from the coupler 103 inevitably includes noises due to the part of the signal light having a predetermined phase difference with respect to the amplified signal light. Due to the presence of noises, a signal read error occurs on the receiving side of the signal light, thereby making it difficult to transmit information accurately.
In a conventional optical fiber amplifier, occurrence of such noises is an inevitable problem, as long as the optical fiber amplifier has the pump light source 102. Since the noises have the same wavelength as that of the signal light, the noises cannot be removed by a conventional filtering apparatus, which removes noises by excluding light having a wavelength different from that of the signal light.